What is depression?

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The low mood of depression is deeper, longer and more unpleasant than the short periods of unhappiness we all have from time to time. It can cause a loss of interest or pleasure in activities, and may interfere with everyday life.

What causes depression?

We are still not sure what causes depression but a combination of factors are thought to play a role. Major life events such as bereavement, redundancy or marital breakdown are all common triggers for depression.

Unhelpful thinking patterns such as imagining the worst - also known as over-generalising or 'catastrophising' ("I fail at everything") - can also contribute to a downward spiral into depression.

Some medical conditions, such as an underactive thyroid (hypothyroidism), can also cause depression. Depression is also a rare side-effect of certain drugs such as beta blockers, which are used to lower blood pressure, and some anti-epileptic drugs.

You can also develop the condition for no single obvious reason. It can come on gradually and you can end up exhausting yourself by trying to struggle on and keep busy rather than seeking help.

The role that brain chemicals play in depression is still poorly understood. However, most experts agree that it is not simply caused by an imbalance in the brain.

Who can get depression?

The short answer is: anyone can develop depression.

However, research suggests there may be a genetic factor involved that makes some more prone to the condition when going through a challenging period. People with a family history of depression are also more likely to experience it themselves. And certain genetic variations on chromosome 3 could play a role in severe and recurring depression - a disorder that affects up to 4% of the population.

The role of anti-depressants

Anti-depressants affect the way that chemicals in the brain work. But the exact role of these chemicals in depression is not fully understood, and anti-depressants do not work for everyone.

It is known that depression changes the balance of various neurotransmitters in the brain. These are the chemical messengers that communicate between brain cells called neurones.

Neurotransmitters send information across tiny spaces called synapses, which are received by a receptor embedded in the next neurone in the chain. The receptors may be oversensitive or insensitive to a specific neurotransmitter, causing their response to its release to be excessive or inadequate.

Anti-depressants gradually increase the levels of 'mood-enhancing' neurotransmitters like serotonin, norepinephrine and dopamine in the brain. This may be why most people need to take the pills for a few weeks before they start to work.

However, according to the Royal College of Psychiatrists some of the benefit is due to the placebo effect. They found that 50 to 65% of people living with depression get better if given an anti-depressant compared to 25 to 30% of those given a 'dummy' pill.

First developed in the 1950s, there are now four main classes of anti-depressants and they affect these brain chemicals in slightly different ways.

The most commonly prescribed are SSRIs, or selective serotonin reuptake inhibitors, which raise the serotonin level in the brain. Serotonin and noradrenaline re-uptake inhibitors (SNRIs) are newer and also affect levels of norepinephrine.

Tricyclic antidepressants and monoamine-oxidase inhibitors (MAOIs) are older and less commonly used as they tend to cause more side-effects.

While anti-depressants can ease the immediate symptoms they do not tackle any root cause, and so are often used in combination with other therapies.

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